Waves and tides

The governing concept is the connection between tidal range and the possible energy available from moving seawater. At full moon, the Sun, Earth, and Moon are approximately aligned. The gravitational effects of the Sun and Moon therefore reinforce one another, producing a spring tide with a greater-than-usual difference between high tide and low tide. A larger vertical range can increase the head, or water-level difference, available for a tidal-energy system, although actual generation also depends on basin shape, flow speed, turbine design, and engineering constraints. Thus option A provides the correct basis. Option B describes neap tide, which occurs near quarter phases. Sea colour and absence of storm wind do not explain the lunar spring-tide increase.

A spring tide has a large difference between high and low water, but near low tide the astronomical water level is lower than near high tide. Storm waves can still cause dangerous run-up, overtopping and erosion, so risk assessment should consider wave energy, the actual water level, storm surge and timing together. Option C is the careful conclusion; low tide does not make storm-wave risk zero.

The governing concept is that tidal-energy plants use the predictable rise and fall of sea level to create a water-level difference and drive turbines. A site is therefore most valuable when its tidal range is large enough to provide substantial potential energy and occurs regularly enough for dependable generation. Option D includes both essential conditions. Wave height describes wind-wave activity, not the main resource for a tidal plant; sea colour and fish sounds have no engineering relevance; and the mere presence of coastal sand does not indicate usable tidal power. Local basin shape, environmental effects, grid access, and construction feasibility would also matter in a real survey, but among the listed choices, large and regular tidal range is unambiguously the best criterion.

The governing concept is coastal sediment transport by wave action. Waves approaching a shore at an angle can produce longshore drift, moving sand along the coast; a seawall reflects, absorbs, or redirects part of the wave energy and can interrupt that pathway. The resulting change may cause deposition on one side, erosion on another, or altered beach width. Thus option A correctly links a human structure with changes in wave energy and sediment movement. Option B is false because waves are major agents of coastal transport. A changed deposition pattern does not mean that tides have ended, and the Moon’s direction is unrelated to construction of the seawall.

The governing concept is the inland influence of tidal motion in an estuary. During high tide, the sea surface and hydraulic pressure at the mouth rise, allowing saline seawater to move upstream, sometimes against the usual seaward river flow. The distance and strength of intrusion depend on tidal range, river discharge, channel shape, and mixing. Option B is correct because it identifies the change in water level and flow direction associated with the tide. Wave oscillation mainly transfers energy and does not by itself explain sustained saltwater penetration. Moon colour is irrelevant, while an undersea earthquake could generate a tsunami but is not the normal cause of a repeated high-tide intrusion.

The governing concept is the threshold of motion and transport capacity. For a given wave energy, coarse grains have greater mass and usually require stronger bed shear to be lifted or carried than fine grains. They may roll, slide, or move only intermittently near the bed, whereas fine particles can be suspended and transported farther when the flow is energetic enough. Therefore option A is correct: equal wave energy does not produce identical movement for different grain sizes. The exact result also depends on grain density, shape, bed roughness, water depth, and wave period. Coarse sediment cannot become wind, does not create tides, and particle size is clearly relevant to erosion, transport, and deposition.

The governing concept is astronomical control of tides. The Moon’s gravity, together with the Sun’s gravity, produces tidal bulges in the ocean system. As Earth rotates, a coastal location moves through these changing bulges and commonly experiences alternating high and low water. The Moon also moves in its orbit, so the interval between successive comparable tides is approximately 12 hours 25 minutes rather than exactly 12 hours; local basin shape and coastline geometry modify the observed timing. Option B is therefore correct because it combines Earth’s rotation with the Moon–Sun gravitational arrangement. Sea colour, fish movement, and a permanent wind direction do not explain the regular astronomical periodicity of tides.

The governing concept is tidal control of navigable depth. A vessel needs enough water beneath its keel not only while entering a port but also while leaving or returning through shallow channels. If entry is planned at high tide and the ship remains until low tide, the available depth may fall below the vessel’s safe-draft requirement, creating a risk of grounding or restricting movement. Option C is correct because it requires checking both stages of the voyage, including tide times, channel depth, vessel draft, and safety margin. Option A considers only half the operation; option B ignores a decisive physical condition; and sea colour has no reliable relation to navigational depth.

The governing concept is tidal range, which is the vertical difference between high-water and low-water levels. A spring tide occurs near the new moon and full moon, when the gravitational effects of the Sun and Moon reinforce one another. Consequently, high tides tend to be higher than average and low tides lower than average, producing an unusually large tidal range. It is not simply a single high-water event. Option A is correct because it defines the condition in terms of the complete high-to-low difference. A neap tide has a smaller range, a wind wave is generated mainly by wind rather than lunar-solar alignment, and sea colour has no connection with the phenomenon.

The governing concept is the balance between wave erosion, rock resistance, and sediment supply. Resistant bedrock can withstand wave attack better than weak material, so erosion is likely to occur more slowly; however, high wave energy can still produce abrasion, hydraulic pressure, and weathering along cracks. Because the coast receives little sediment, there is not enough mobile material to support extensive beach deposition, especially where energetic waves remove particles. Option B correctly combines both conditions: erosion is reduced rather than impossible, and deposition is limited by sediment scarcity. High wave energy does not automatically create abundant deposition. Tides do not stop because of local rock strength, and a coast with these characteristics will certainly not cause the entire sea to dry up.

The governing concept is the distinction between wave motion and current motion. In a surface wave, energy and the wave form travel forward, while water particles generally move in orbital or oscillatory paths and may show little net forward displacement in deep water. A tide-driven current, by contrast, is a net movement of water through a channel or across a coast, often horizontally toward land during flood tide and seaward during ebb tide. Option C states this essential contrast. Waves are not colour changes and are not stationary; tidal currents are not lunar eclipses. In shallow water, wave particles and sediment may acquire net transport, but the basic school-level distinction remains oscillation associated with waves versus sustained flow associated with currents.

The governing concept is the interaction of astronomical forcing with local coastal geometry. At full moon, the Sun, Earth, and Moon are approximately aligned, so solar and lunar tidal forces reinforce one another and produce a spring tide with a larger tidal range. If a bay narrows toward its head or funnels water through a restricted entrance, the same volume of moving water may be concentrated, increasing current speed. Option D correctly combines the large spring-tide range with the amplifying effect of bay shape. The Moon’s influence has not ended; wind may contribute to water movement but cannot alone explain this astronomical timing; and tides are fundamentally movements of ocean water.

An expert response should synthesize the factors rather than treating one observation as a complete explanation. Tide timing indicates the available water level and the timing of inundation or currents. Wave energy indicates the capacity for erosion, transport and breaking. Rock nature supplies resistance: hard, coherent rock responds differently from weak or fractured material. Sediment availability determines whether transported particles can be supplied, moved or deposited. Linking these in the order of water level, energy, resistance and material creates a clear causal chain from conditions to coastal result, so option B is correct. Option A loses important evidence, option C is irrelevant, and option D incorrectly labels every multi-factor coastal situation as a tsunami.